Liquid-phase oxidation of an aromatic alkyl hydrocarbon to an aromatic carboxylic acid is a highly exothermic chemical reaction. Volatilizable aqueous acidic solvents are used to contain the reaction mixture and to dissipate the heat of reaction. The oxidation of aromatic alkyl hydrocarbons in the liquid phase to form aromatic carboxylic acids is generally performed in a vented, well-mixed oxidation reactor, with a substantial portion of the heat generated by the exothermic oxidation reaction being removed by refluxing a portion of the aqueous solvent and aromatic alkyl hydrocarbon contained within the reactor.
The optical quality of the produced aromatic carboxylic acid is dependent to a large extent on the mixing that takes place in the oxidation reactor inasmuch as reactor mixing disperses the reactants, promotes oxygen mass transfer, and keeps the produced aromatic carboxylic acid in suspension. Uniform dispersion of the reactants and the efficiency of oxygen mass transfer influence the oxidation rate of the aromatic alkyl hydrocarbon and produced intermediate compounds, as well as the formation of undesirable optical impurities and by-products. This, in turn, influences the optical quality of produced aromatic carboxylic acid, reactor burning, and reaction yield. Accordingly, there are significant economic incentives for improving reactor mixing performance.
The materials vaporized as a result of the heat generated in the exothermic reaction, together with unreacted oxygen and other noncondensable components that may be present, pass upwardly through the reactor and are withdrawn from the reactor at a point above the reaction mixture liquid level in the reactor. The vapors are passed upwardly and out of the reactor to an overhead reflux condenser system where the vaporized solvent, water and aromatic alkyl are condensed. The condensed materials, now at a temperature less than the reactor contents' temperature, are returned to the reactor by gravity. The noncondensable gases, carried along with the vaporized reactor material, are vented.
In operation, the reactor is fed by a liquid feed stream containing the aromatic alkyl hydrocarbon, aqueous acidic solvent and an oxidation catalyst. An oxygen-containing gas is separately introduced into the reactor for oxidizing the aromatic alkyl hydrocarbon to the aromatic carboxylic acid in the presence of the catalyst.
The reaction mixture contained in the reactor includes a suspension of the produced aromatic carboxylic acid. Since the reaction mixture contains solid-phase and liquid-phase components, as well as the continuously introduced oxygen-containing gas, vigorous stirring of the reactor contents is necessary to maintain the desired uniform reactor conditions and to obtain a high quality product. The vigorous stirring is costly in terms of power input; however. Also, even with high power input the mixing effectiveness of prior systems has not been as good as desired.
In one system for the production of an aromatic carboxylic acid, the reaction takes place in a vertically disposed elongated vessel having a substantially cylindrical sidewall and having an agitator mounted for rotation within the vessel on a shaft at about the axis of the vessel. The agitator has dual impellers with an upper mixing element, or impeller, in the form of a 4-blade disc turbine at an intermediate location on the shaft and a lower mixing element, or impeller in the form of a 4-blade turbine with pitched blades at the lower end of the shaft.
The reactor in this system has four vertical baffles substantially evenly spaced on the cylindrical vessel walls. Each baffle is of rectangular cross-section and has a width of about 1/12 of the vessel inside diameter. Each baffle is spaced from the vessel wall by a clearance of about 1/100 of the vessel diameter. Gas introduction in such a system heretofore has been at a level somewhat higher than the level of the lower impeller.
The present invention provides an oxidation reactor having enhanced mixing performance. Gas hold-up in the reactor is increased and the power required to maintain solid reaction products in suspension is substantially reduced. As a result, an aromatic carboxylic acid product of relatively higher quality can be obtained.